The present invention relates to a method of producing an olefin thermoplastic resin composition of good impact-resistance and, particularly, to a method of producing a thermoplastic resin composition which comprises an olefin copolymer rubber forming a fixed phase structure and a crystalline polypropylene resin.
Because the crystalline olefin resin has a poor impact-resistance, a rubber component as an impact-modifier is mixed with it in order to overcome the above defect and to improve the impact-resistance. It has been known that a high impact property can be obtained by the method in which the rubber component is added to the crystalline polypropylene or a mixture of crystalline polypropylene and polyethylene particularly.
If a large quantity of olefin copolymer rubber component as the impact-modifier was melt-blended with the crystalline polypropylene, the impact-strength might be improved but the mechanical strength, elasticity, heat-resistance and flowability would be decreased, together with the additional disadvantages that the process properties, such as injection molding, would deteriorate. On the other hand, if a relatively small quantity of rubber component were blended so as to improve the mechanical strength, heat-resistance and fluidity, such defects as poor impact-strength, elongation and toughness, would be encountered.
In order to overcome the above problems, U.S. Pat. No. 4,212,787 discloses a method which comprises partially curing an olefin copolymer rubber under shear stress and then diluting it with an olefin resin of good fluidity.
The problems resulting from the equipment needed for the curing and diluting processes and the complexity of the process can be pointed out as defects.
The present inventors have known that if the phase structure of the crystalline polypropylene and the olefin copolymer rubber form a cellular structure, the flowability, which is important in injection molding, could be improved and then the low impact-strength and heat-resistance could be also improved.
The cell structure publicly known up to now can be made only in a crystalline polypropylene/polyethylene/ethylene-propylene copolymer rubber system and if these components are blended in the proper proportions, the cell structure can be achieved by their compatibility. But it has been known that a crystalline polypropylene/ethylene-propylene copolymer rubber system, except with a part of a crystalline ethylene-propylene copolymer resin, cannot form the cell structure according to the limited compatibility of these materials.
One known solution to this problem has been to use a cross-linking agent such as a peroxide and to blend on melting so as to form the cell structure of ethylene-propylene copolymer rubber together with a crystalline polypropylene resin. This procedure, however, raises another problem. The problems concerning physical properties like decreased elongation, toughness at a low temperature, etc., simultaneously occur in the process for improving the flowability through the dilution procedure of the previously cured elastomer component. Also, the restrictions according to the apparatus used during the dispersion process and the usage of crystalline polypropylene having good flowability are pointed out as problems.